The present invention relates generally to the PES (Position Error Signal) Certification Test for defects of servo bursts in a disc drive. More particularly, the present invention relates to a method of screening oscillatory PES with 1.7 kHz harmonic during the PES Test.
The present test used for screening out bad tracks in a disc drive is the PES (Position Error Signal) Test. In the PES Test, every data track is scanned and tested for any servo defect. During the test, any defects concerning the servo will be reported and appropriate action will be taken by the algorithm. Depending on the type of servo error, action taken can be either zapping the defective burst or skipping the track containing the defect. Therefore, it is important that all servo defects must be detected before the Defect Scan Test is performed. In the Defect Scan Test, the read and write operations are performed, where the position is dependent on the PES data. However, there is the presence of a 1.7 kHz NRRO (Non-Repeatable Run Out) harmonic resonance due to windage excitation of the suspension at the area of the head assembly on the actuator arm. This has been proven by drive level LDV (Laser Doppler Vibrometer) analysis and from which it has shown that the HGA (Head Gimbal Assembly) have similar vertical motion, but bad HGAs transmit more of this vertical motion into off track motion. And, the focus on preventing windage excitation on the sensitive pre-load bend region of the actuator arm with wind diffusion wedge, damper and load beam can show that 1.7 kHz resonance on the PES can be greatly reduced. However, there is a need for screening the 1.7 kHz harmonic when performing PES Test due to the reason that windage excitation is possible and all servo tracks must be ensured good for read and write operations during Defect Scan Test. This is because the windage excitation can happen at the time of servo-writing whereby the servo pattern is written onto each platter of the HDA (Head Disc Assembly). When this happens, the 1.7 kHz resonance will be embedded and written in the servo pattern. Hence, in such a way, the 1.7 kHz resonance has resulted to be RRO (Repeatable-Run Out) problem as it is already inherent as a write-in. The PES test is based on the amplitude change of the PES of a servo burst from its on-track value, if the amplitude change exceeds the PES threshold criterion value, that particular servo burst would be considered as a bad servo and will be zapped. According to this testing criterion, as long as the PES has not made any huge jump in amplitude away from the on-track value, (i.e., the head is not too far away from the on-track position), it is said that the PES will be considered good enough for any read and write operation. However, the current PES Test algorithm, shows that there are drives that have passed the PES Test but failed the Defect Scan Test with servo off-track error code. This is because the drive code setting criterion is dependent on the position and velocity error code. For this particular case, the 1.7 kHz resonance will definitely have higher velocity error. As a result, it is clear that the method testing for PES threshold will not be sufficient in PES Test. In FIG. 8, FIG. 9, FIG. 10 and FIG. 11, the PES measurement obtained from tracks with and without 1.7 kHz harmonic for the above explanation is shown. FIG. 8 and FIG. 9 depicts waveform of oscillatory PES with 1.7 kHz harmonic along with that from a good track. The PES spectrums in FIG. 10 and FIG. 11 show the difference in amplitude of the PES between a good track and a track with inherent 1.7 kHz harmonic. It is observed that the amplitude of the 1.7 kHz harmonic is 1.167 Vpp (Peak to Peak Voltage) (7.78%) while the amplitude of the good track is 0.703Vpp (4.68%). The CSPT (Certificate Sequence Parameter Table) criterion for PES threshold is 9%. Therefore, the track can still pass the PES Test and it can be that PES threshold testing is not sufficient for screening any track containing 1.7 kHz resonance.
There remains a need for an improved method for scanning tracks that are defective due to the presence of high frequency harmonics. It will be evident from the following description that the present invention offers this and other advantages
The present invention proposes a new algorithm to be added to the existing mechanism in detecting bad tracks that contain 1.7 kHz (Repeatable-Run Out) RRO resulting from written-in PES oscillatory and thus, allowing PES Test to screen-out these tracks.
According to one aspect of the invention, there is provided a method for screening tracks on a magnetic disc in a disc drive using Position Error Signal (PES) velocity to determine the presence of high frequency PES component, the method which include the algorithms to obtain a summation of all absolute PES velocities on a single track in a revolution. The method also include an algoriothm to obtain an average PES velocity using the summation of all absolute PES velocities for a single revolution. Another algorithm provided by the method is to obtain an average PES velocity for a multiple number of revolutions of the track using the average PES velocity for each revolution and compare this average PES velocity with a pre-set standard criterion value.
Another aspect of the present invention provides for the algorithms to be repeated with the number of revolutions varied.
Another aspect of the present invention enables the standard criterion value to be pre-determined empirically.
The method in the present invention can magnify the accumulated value, which enables a good track to be differentiated from a track with 1.7 kHz resonance.
The present invention also can be implemented in a disc drive as a program for a computer system to perform a method for screening PES velocity to determine the presence of high frequency PES component.